Our universe might be really, really big — but finite. Or it might be infinitely big.

Both cases, says physicist Brian Greene, are possibilities, but if the latter is true, so is another posit: There are only so many ways matter can arrange itself within that infinite universe. Eventually, matter has to repeat itself and arrange itself in similar ways. So if the universe is infinitely large, it is also home to infinite parallel universes.

Does that sound confusing? Try this:

Think of the universe like a deck of cards.

"Now, if you shuffle that deck, there's just so many orderings that can happen," Greene says. "If you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe and only a finite number of complexions of matter, the way in which matter arranges itself has to repeat."

Greene, the author of The Elegant Universe and The Fabric of the Cosmos, tackles the existence of multiple universes in his latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.

Recent discoveries in physics and astronomy, he says, point to the idea that our universe may be one of many universes populating a grander multiverse.

"You almost can't avoid having some version of the multiverse in your studies if you push deeply enough in the mathematical descriptions of the physical universe," he says. "There are many of us thinking of one version of parallel universe theory or another. If it's all a lot of nonsense, then it's a lot of wasted effort going into this far-out idea. But if this idea is correct, it is a fantastic upheaval in our understanding."

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How Quantum Mechanics And General Relativity Play A Part

Greene thinks the key to understanding these multiverses comes from string theory, the area of physics he has studied for the past 25 years.

In a nutshell, string theory attempts to reconcile a mathematical conflict between two already accepted ideas in physics: quantum mechanics and the theory of relativity.

"Einstein's theory of relativity does a fantastic job for explaining big things," Greene says. "Quantum mechanics is fantastic for the other end of the spectrum — for small things. The big problem is that each theory is great for each realm, but when they confront each other, they are ferocious antagonists, and the mathematics falls apart."

String theory smooths out the mathematical inconsistencies that currently exist between quantum mechanics and the theory of relativity. It posits that the entire universe can be explained in terms of really, really small strings that vibrate in 10 or 11 dimensions — meaning dimensions we can't see. If it exists, it could explain literally everything in the universe — from subatomic particles to the laws of speed and gravity.

So what does this have to do with the possibility that a multiverse exists?

"There are a couple of multiverses that come out of our study of string theory," Greene says. "Within string theory, the strings that we're talking about are not the only entities that this theory allows. It also allows objects that look like large flying carpets, or membranes, which are two dimensional surfaces. And what that means, within string theory, is that we may be living on one of those gigantic surfaces, and there can be other surfaces floating out there in space."

That theory, he says, might be testable in the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research.

"If we are living on one of these giant membranes, then the following can happen: When you slam particles together — which is what happens at the LHC — some debris from those collisions can be ejected off of our membrane and be ejected into the greater cosmos in which our membrane floats," he says. "If that happens, that debris will take away some energy. So if we measure the amount of energy just before the protons collide and compare it with the amount of energy just after they collide, if there's a little less after — and it's less in just the right way — it would indicate that some had flown off, indicating that this membrane picture is correct."

Greene explains that when he began studying string theory and parallel universes, it wasn't because he could one day measure energy at CERN or develop new mathematical equations. He simply liked the idea, he says, of studying something on such a large scale.

"We're trying to talk about not just the universe but perhaps other universes — but all within a logical framework that allows us to make some definitive statements," he says. "To me, that's enormously exciting, to step outside the everyday and really look at the universe, within these mathematical terms, on its grandest scales."

If, when I was growing up, my room had been adorned with only a single mirror, my childhood daydreams might have been very different. But it had two. And each morning when I opened the closet to get my clothes, the one built into its door aligned with the one on the wall, creating a seemingly endless series of reflections of anything situated between them. It was mesmerizing. I delighted in seeing image after image populating the parallel glass planes, extending back as far as the eye could discern. All the reflections seemed to move in unison — but that, I knew, was a mere limitation of human perception; at a young age I had learned of light's finite speed. So in my mind's eye, I would watch the light's round-trip journeys. The bob of my head, the sweep of my arm silently echoed between the mirrors, each reflected image nudging the next. Sometimes I would imagine an irreverent me way down the line who refused to fall into place, disrupting the steady progression and creating a new reality that informed the ones that followed. During lulls at school, I would sometimes think about the light I had shed that morning, still endlessly bouncing between the mirrors, and I'd join one of my reflected selves, entering an imaginary parallel world constructed of light and driven by fantasy. It was a safe way to break the rules.

To be sure, reflected images don't have minds of their own. But these youthful flights of fancy, with their imagined parallel realities, resonate with an increasingly prominent theme in modern science — the possibility of worlds lying beyond the one we know. This book is an exploration of such possibilities, a considered journey through the science of parallel universes.

Universe and Universes

There was a time when "universe" meant "all there is." Everything. The whole shebang. The notion of more than one universe, more than one everything, would seemingly be a contradiction in terms. Yet a range of theoretical developments has gradually qualified the interpretation of "universe." To a physicist, the word's meaning now largely depends on context. Sometimes "universe" still connotes absolutely everything. Sometimes it refers only to those parts of everything that someone such as you or I could, in principle, have access to. Sometimes it's applied to separate realms, ones that are partly or fully, temporarily or permanently, inaccessible to us; in this sense, the word relegates ours to membership in a large, perhaps infinitely large, collection.

With its hegemony diminished, "universe" has given way to other terms introduced to capture the wider canvas on which the totality of reality may be painted. Parallel worlds or parallel universes or multiple universes or alternate universes or the metaverse, megaverse, or multiverse — they're all synonymous and they're all among the words used to embrace not just our universe but a spectrum of others that may be out there.

You'll notice that the terms are somewhat vague. What exactly constitutes a world or a universe? What criteria distinguish realms that are distinct parts of a single universe from those classified as universes of their own? Perhaps someday our understanding of multiple universes will mature sufficiently for us to have precise answers to these questions. For now, we'll use the approach famously applied by Justice Potter Stewart in attempting to define pornography. While the U.S. Supreme Court wrestled mightily to delineate a standard, Stewart declared simply and forthrightly, "I know it when I see it."

In the end, labeling one realm or another a parallel universe is merely a question of language. What matters, what's at the heart of the subject, is whether there exist realms that challenge convention by suggesting that what we've long thought to be the universe is only one component of a far grander, perhaps far stranger, and mostly hidden reality.

During the last half century, science has provided ample ways in which this possibility might be realized.

Varieties of Parallel Universes

A striking fact (it's in part what propelled me to write this book) is that many of the major developments in fundamental theoretical physics — relativistic physics, quantum physics, cosmological physics, unified physics, computational physics — have led us to consider one or another variety of parallel universe. Indeed, the chapters that follow trace a narrative arc through nine variations on the multiverse theme. Each envisions our universe as part of an unexpectedly larger whole, but the complexion of that whole and the nature of the member universes differ sharply among them. In some, the parallel universes are separated from us by enormous stretches of space or time; in others, they're hovering millimeters away; in others still, the very notion of their location proves parochial, devoid of meaning. A similar range of possibility is manifest in the laws governing the parallel universes. In some, the laws are the same as in ours; in others, they appear different but have shared a heritage; in others still, the laws are of a form and structure unlike anything we've ever encountered. It's at once humbling and stirring to imagine just how expansive reality may be.

Some of the earliest scientific forays into parallel worlds were initiated in the 1950s by researchers puzzling over aspects of quantum mechanics, a theory developed to explain phenomena taking place in the microscopic realm of atoms and subatomic particles. Quantum mechanics broke the mold of the previous framework, classical mechanics, by establishing that the predictions of science are necessarily probabilistic. We can predict the odds of attaining one outcome, we can predict the odds of another, but we generally can't predict which will actually happen. This well-known departure from hundreds of years of scientific thought is surprising enough. But there's a more confounding aspect of quantum theory that receives less attention. After decades of closely studying quantum mechanics, and after having accumulated a wealth of data confirming its probabilistic predictions, no one has been able to explain why only one of the many possible outcomes in any given situation actually happens. When we do experiments, when we examine the world, we all agree that we encounter a single definite reality. Yet, more than a century after the quantum revolution began, there is no consensus among the world's physicists as to how this basic fact is compatible with the theory's mathematical expression.

Excerpted from The Hidden Reality by Brian Greene Copyright 2011 by Brian Greene. Excerpted by permission of Knopf, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.